This invention relates to measuring equipment for determining terrestrial refraction when making geodetic measurements. The invention is particularly useful in geodetic engineering for staking out by means of an electronic tachymeter, when designed heights must be trigonometrically transferred. The invention is particularly useful in precision tachymetry, for precise levelling and for the control of building machinery, for example by means of an active collimating ray (laser beam).
It is known that the refraction of air layers close to the ground cannot be provided with the reliability needed for practical use. The cause of refraction is the variable state of the airlayers. Many studies have shown that a strong correlation exists between the deviation of the collimating ray and the vertical gradient of temperature, so that the coefficient of refraction can be determined mainly as a function of the temperature gradient.
When transferring heights trigonometrically, the influence of the refraction may be substantially eliminated by simultaneously measuring the opposing angles of zenith. A residual error, though, may still occur as a consequence of varying coefficients of refraction at both endpoints. In the unilateral trigonometrical transfer of heights, as may be the case when staking out, for precision tachymetry and for the regulation of building machinery by means of an active collimating ray, the values of measurement are subject to refraction. In order to determine the effective coefficient of refraction as nearly as possible, the gradient of temperature along the path of light between the geodetic instrument and the sighting point should be known. Close to the earth's surface the path of light may be defined approximately as an arc and the gradient of temperature may be calculated based upon measurements of difference of temperature between points of differing height near the base of the instrument and, in order to improve the results, also additively by measuring differences of temperature at points near the sighting point. Additional measurements of differences of temperature at discrete points in the direction of the collimating ray have been undertaken in scientific studies. Such measurement arrangements employing several stations for the measurement of temperature differences between the basepoint and the point of sight are too expensive for practical purposes, because they must be constantly reconstructed since the points of sight are usually located in various directions.
It is known that variations of refraction occur, depending on the distance from the point of sight. Thus an observer may visibly expect that the accuracy of a geodetic instrument decreases with increasing distance from the geodetic instrument when averaging different adjacent local coefficients of refraction. In other words, for the determination of a probable coefficient of refraction, the local coefficients of refraction at the point of view influence the deviation of the collimating ray much more than those further afield. The known method of determination of angles of refraction by using the dispersion effect requires the use of very complicated instruments and can scarcely be of any use in airlayers close to the ground, due to the turbulence in this region.